Hi bpf@!
Tl;DR:
BPF programs built with `clang`'s `-g -Wa,--compress-debug-sections`
fail when processed by `bpftool` on section alignment checks as:
$ cat src/core/bpf/socket_bind/bpf.c
struct socket_bind_map_t { int value; };
struct socket_bind_map_t sd_bind_allow __attribute__((section(".maps"), used));
$ clang-16 -target bpf -fno-stack-protector -c src/core/bpf/socket_bind/bpf.c -o bpf.unstripped.o -g -Wa,--compress-debug-sections
$ bpftool gen object bpf.o bpf.unstripped.o
libbpf: ELF section #9 has inconsistent alignment addr=8 != d=4 in bpf.unstripped.o
Error: failed to link 'bpf.unstripped.o': Invalid argument (22)
This happens only when `bpftool` is a 32-bit ELF binary. 64-bit
`bpftool` seems to work as expected.
More words:
Here is my understanding of the failure. Without the
`-Wa,--compress-debug-sections` option `clang` generates byte-aligned
uncompressed `.debug` sections:
$ clang -target bpf -fno-stack-protector -c src/core/bpf/socket_bind/bpf.c -o bpf.unstripped.o -g
$ readelf -SW bpf.unstripped.o
There are 19 section headers, starting at offset 0x530:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000471 0000b8 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000000 00 AX 0 0 4
[ 3] .maps PROGBITS 0000000000000000 000040 000004 00 WA 0 0 4
[ 4] .debug_abbrev PROGBITS 0000000000000000 000044 00004c 00 0 0 1
[ 5] .debug_info PROGBITS 0000000000000000 000090 00003d 00 0 0 1
[ 6] .rel.debug_info REL 0000000000000000 000380 000040 10 I 18 5 8
[ 7] .debug_str_offsets PROGBITS 0000000000000000 0000cd 000024 00 0 0 1
[ 8] .rel.debug_str_offsets REL 0000000000000000 0003c0 000070 10 I 18 7 8
[ 9] .debug_str PROGBITS 0000000000000000 0000f1 000080 01 MS 0 0 1
[10] .debug_addr PROGBITS 0000000000000000 000171 000010 00 0 0 1
[11] .rel.debug_addr REL 0000000000000000 000430 000010 10 I 18 10 8
[12] .BTF PROGBITS 0000000000000000 000184 000099 00 0 0 4
[13] .rel.BTF REL 0000000000000000 000440 000010 10 I 18 12 8
[14] .debug_line PROGBITS 0000000000000000 00021d 000043 00 0 0 1
[15] .rel.debug_line REL 0000000000000000 000450 000020 10 I 18 14 8
[16] .debug_line_str PROGBITS 0000000000000000 000260 000041 01 MS 0 0 1
[17] .llvm_addrsig LOOS+0xfff4c03 0000000000000000 000470 000001 00 E 18 0 1
[18] .symtab SYMTAB 0000000000000000 0002a8 0000d8 18 1 8 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
D (mbind), p (processor specific)
Note how `.debug_str` has `alignment=1` here. Loading of such a section
has no problems on x86_64 or i686.
But when we add `-Wa,--compress-debug-sections` some debug sections
become aligned 8 bytes (usual `ELF64`) alignment:
$ clang -target bpf -fno-stack-protector -c src/core/bpf/socket_bind/bpf.c -o bpf.unstripped.o -g -Wa,--compress-debug-sections
$ LANG=C readelf -SW bpf.unstripped.o
There are 19 section headers, starting at offset 0x530:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000471 0000b8 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000000 00 AX 0 0 4
[ 3] .maps PROGBITS 0000000000000000 000040 000004 00 WA 0 0 4
[ 4] .debug_abbrev PROGBITS 0000000000000000 000044 00004c 00 0 0 1
[ 5] .debug_info PROGBITS 0000000000000000 000090 00003d 00 0 0 1
[ 6] .rel.debug_info REL 0000000000000000 000380 000040 10 I 18 5 8
[ 7] .debug_str_offsets PROGBITS 0000000000000000 0000cd 000024 00 0 0 1
[ 8] .rel.debug_str_offsets REL 0000000000000000 0003c0 000070 10 I 18 7 8
[ 9] .debug_str PROGBITS 0000000000000000 0000f1 000086 01 MSC 0 0 8
[10] .debug_addr PROGBITS 0000000000000000 000177 000010 00 0 0 1
[11] .rel.debug_addr REL 0000000000000000 000430 000010 10 I 18 10 8
[12] .BTF PROGBITS 0000000000000000 000188 000099 00 0 0 4
[13] .rel.BTF REL 0000000000000000 000440 000010 10 I 18 12 8
[14] .debug_line PROGBITS 0000000000000000 000221 000043 00 0 0 1
[15] .rel.debug_line REL 0000000000000000 000450 000020 10 I 18 14 8
[16] .debug_line_str PROGBITS 0000000000000000 000264 000041 01 MS 0 0 1
[17] .llvm_addrsig LOOS+0xfff4c03 0000000000000000 000470 000001 00 E 18 0 1
[18] .symtab SYMTAB 0000000000000000 0002a8 0000d8 18 1 8 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
D (mbind), p (processor specific)
Note how `.debug_str` alignment changed from 1-byte to 8-byte alignment.
AFAIU 8-byte alignment of compressed section for `ELF64` files is expected:
compressed sections all have a small header that consistes of a few
64-bit values (compression type, uncompressed size, uncompressed data
alignment).
Thus the binary loads as expected by `x86_64` `bpftool` and fails when
loaded by `i686` `bpftool`.
Should `bpftool` work in this scenario? Or compressed sections are not
supported on 32-bit hosts?
It feels like debugging sections with strings should be easily
decompressable on any host type.
Thanks!
--
Sergei
